Eridium

Eridium, not to be confused with Iridium, is an element first discovered by the Eridians, but sprouted massively on Pandora after the Vault of the Destroyer was opened and Infinite energy spread from its home dimension into Midgard.

Eridium is a solidified form of pure Infinite energy, formed naturally in the Infinite and occurs in other dimensions due to excess Infinite energy naturally attempting to resolve itself. It was first used by the Eridians, allowing them to access the Infinite, to power the Vaults and the Vault Key, along with raid boss summoning posts and portals to the Infinite.

Eridian Discovery
Eridium was supposedly first discovered by the Eridians, hence the namesake. While it is unknown how exactly they found Eridium, it was shown on tomb drawings that they may have found it sometime after expanding across the planet of Pandora, in small clusters in very small caves. Upon leaving Midgard, they left behind chunks of Eridium which were found and could be used to charge Eridian weapons or create Eridian relics.

Eridium and Sirens
Due to the Sirens being created by Eridians, Sirens can absorb Eridium, thereby bolstering their connection to the Infinite enhance their powers. However, this has serious side effects. While Eridium increases a Siren's powers and makes them more powerful, even increasing their natural healing to the point of superhuman regeneration, the 'high' of the power boost has the potential to be addictive. Sirens that are given, willingly or forcefully, a constant flow of Eridium to absorb become unable to live without it: the Eridium essentially becomes their life support. Should the constant flow of Eridium stop, the Siren will die.

This is due to absorbing too much Infinite energy, as well as the absorption of energies which conflict while not operating harmoniously in the Infinite.

Maya is the only known Siren unable to use Eridium to charge herself, while Matt, a half-Siren, cannot either, although he may not need to as he can generate Infinite energy anyway.